Allergic diseases among children: nutritional prevention and intervention

Background

Allergic diseases comprise a genetically heterogeneous group of chronic, immunomediated diseases^1,2 that mainly involve bronchial asthma, allergic rhinitis, atopic dermatitis, food allergy, and acute urticaria. They are more prevalent among children than adults. Worldwide, respiratory allergic diseases alone, namely asthma and allergic rhinitis, affect nearly 700 million subjects.³ It has been clearly reported that the prevalence of these diseases has been on the rise for the last few decades, but at different rates, in various areas of the world.^3,4 Currently, bronchial asthma is considered the most common chronic, noninfectious condition among children.⁴ In some industrialized countries, the prevalence of asthma is close to 35%–40%, whereas it is less than 5% in other communities;⁵ furthermore, relatively new reports have shown that the prevalence of asthma is increasing in many low- and middle-income nations.^4,6

The impact of allergic diseases is tremendous on affected individuals, their families, and societies. They adversely affect quality of life and increase the rate of comorbid conditions and risk of death, as noticed in asthma.^3,7 In addition, the economic burden of these diseases is considerable. This is usually related to the substantial direct medical cost (emergency department visits, physician’s office visits, hospitalizations, diagnostic laboratory and radiological workup, and other modalities of therapy) and indirect medical costs (numerous absences from labor or school, reduced productivity, and diminished school performance).^3,7–9

Allergic diseases are intricate diseases resulting from the interaction of genetic and environmental factors.¹⁰ The latter include infectious agents (human rhinoviruses, respiratory syncytial virus, and mycoplasma), allergens (house dust mites, pollens, pets, and molds), pollutants, and medication exposure.^11–13

During the last 2 decades, primary prevention of allergic diseases and asthma (prevention of immunological sanitization, development of IgE antibodies) has proved to be a more effective strategy than secondary prevention (preventing the development of an allergic disease following sensitization) or tertiary prevention (treatment of asthma of allergic diseases).¹⁴ As a strategy, primary prevention of allergic diseases includes 1) allergen avoidance, 2) restoration of gut microbiota–intestinal immunity relationship, 3) dietary pattern, 4) intake of modified dietary proteins in early life, and 5) micronutrients.

The Cochrane Library, Embase, Medline, and Google Scholar databases were searched in March 2015 (from inception to March 2015) for randomized controlled trials (RCTs), quasi-RCTs, and review articles that were published in English and Spanish. In addition, we searched the references of the identified articles for additional articles. We used a wide variety of different combinations of the following terms: allergy, primary, prevention, children, mothers, diet, milk, formula, breast-feeding, prebiotics, probiotics, synbiotics, Mediterranean, micronutrients, and fatty acids. We identified and reviewed 254 manuscripts, but filtered to 153 references (Figure 1). This review paper focuses on primary prevention of allergic diseases in children.

Figure 1 Flow diagram of selection of literature.

Restoration of gut microbiota–intestinal immunity relationship

Since the allergy-high-risk infant (born to one or more allergic parent, and/or allergic sibling) is healthy, the strategy of preventing immunity alteration is aimed at the preservation of the “beneficial” gut microbiota in the newborn, and thus preventing deviations of the local intestinal immunity and consequently the systemic immunity to the proallergic side. To understand the rationale behind this preventive strategy, the bacterial colonization of gut, infant intestinal microbiota–intestinal immune interaction, and imbalances in the composition of the intestinal microbiota leading to allergic diseases have to be explored.

Gut microbiota, intestinal immunity, and allergic diseases

Microbial gut colonization typically commences at the time of birth, and is influenced by the load (inoculum) of the first maternal microbiota, type of delivery (cesarean section vs vaginal delivery), feeding practices (formula-feeding vs breast-feeding), and antimicrobial use.^15–17 The maternal intestinal microbiota clearly determines the type of infant’s intestinal microbiota in the first few months of life.¹⁸ Moreover, it takes a few months for the bacterial species to become steadily consistent, despite the early life complete colonization.^16,19 At the age of 1 month, infants born via cesarean section demonstrate similar numbers of gut bacteria to those who were vaginally delivered, but with higher rates of Clostridium spp., Klebsiella spp., Enterobacteriaceae, Bacteroides spp., Bifidobacterium spp., and Escherichia coli.^16,20,21 This effect extends beyond the age of 6 months in infants born via cesarean section,²² increasing the risk of atopy, asthma, and allergic rhinitis.²³ In normal breast-fed infants, Bifidobacterium spp. predominate the gut microbiota (60%–70%), compared to formula-fed infants, where Bacteroides and Clostridium spp. and Enterobacteriaceae prevail.^16,21 Perturbations in the intestinal microbial profile result in the reduction of the predominance of Bifidobacterium spp. or rise of unbeneficial microbiota, leading to increased risk of allergies, infections,¹⁹ and other diseases.²⁴

In the newborn period, the gut microbiota plays a crucial role in promoting and maintaining the mucosal immune system, both in terms of its physical factors and function, and in maintaining a very well-balanced immune response. Gut-associated mucosal lymphoid tissue becomes reactive to pathogenic bacteria but tolerant to “beneficial” bacteria. Moreover, the intestinal microbiota plays an important role in the development of tolerogenic dendritic cells from the mesenteric lymph nodes of the gut-associated mucosal lymphoid tissue and in the production of secretory IgA.^25,26

The intestinal epithelia express various pathogen-recognition receptors, including Toll-like receptors and nucleotide-binding oligomerization domain-like receptors that activate immune response against pathogens. Since pathogenic bacteria and gut commensals express many of these pathogen-associated molecular patterns, there is tight control on the immune-response mechanisms so that it will recognize and specifically respond to pathogens, but at the same time remain tolerant to commensals. These mechanisms involve intestinal epithelial cells, Toll-like receptors, dendritic cells, and T-regulatory (T_reg) cells.²⁷

It is known that T-helper (Th)-2 cells are characterized by their production of IL-4, IL-5, IL-9, and IL-13, which contribute to the development of and maintenance of allergic inflammatory process, while Th1 cells produce TNFα and IFNγ, which contribute in the modulation of cell-mediated immunity.^27,28

T_reg cells, however, maintain immunological tolerance due to their immunomodulatory or immunosuppressive capabilities, and are key players in regulating immune response in nondisease states.²⁷ Changes in T_reg numbers or functions are usually associated with development of allergic disease (Figure 2). Basic animal studies on germ-free mice revealed that in the absence of microbial colonization, T-cells produced more Th2 cytokines. This Th2-cell predominance was corrected with colonization of germ-free mice with Bacteroides fragilis alone, which promoted Th1 response and restored the Th1/Th2 imbalance.²⁸ Evidence from clinical studies also demonstrated that probiotic bacteria reduced Th2 cytokine patterns while promoting Th1 response.²⁹ The intestinal microbiota have marked immunomodulatory effects that are essential in maintaining immune tolerance. It has been proposed that certain diseases, such as eczema, asthma, allergic rhinitis, type 1 diabetes mellitus, and inflammatory bowel diseases, are linked to the dysregulation of the development of the intestinal mucosal defense system.³⁰

Figure 2 In early infancy, colonization with beneficial flora stimulates the intestinal immunity to develop immunotolerance through intraintestinal epithelial cells and dendritic cells and their surface markers, leading to balanced Th1/Th2 immune response. Note: Perturbations of this process (eg, by cesarean section delivery) disrupts microbiota, deviates immune response, and induces allergic response. Abbreviation: Th, T-helper.

The composition of intestinal microflora differs between atopic and healthy infants.²⁰ Atopic children have lower counts of Lactobacillus, Bifidobacterium, and Bacteroides spp.³¹ A recent systematic review by Melli et al³² on the role of intestinal microbiota and allergic diseases among children revealed that in allergic children, early life microbiota was of lower diversity, predominantly Firmicutes; the same group of children also showed a higher amount of Bacteroidaceae and anaerobic bacteria, particularly B. fragilis, E. coli, Clostridium difficile, Bifidobacterium catenulatum, and B. bifidum, and fewer B. adolescentis, B. bifidum, and Lactobacillus. Well-conducted clinical trials have provided good data on the preventive benefit of probiotics (Lactobacillus or Bifidobacterium) in pregnant women and their infants in reducing the risk of allergic diseases, especially atopic dermatitis.^33,34

Breast milk constitutes a major source of support to gut colonization, due to its Bifidobacterium content and its composition of a large amount of galactooligosaccharides (GOS), which selectively accelerate the growth of bifidobacteria.^35,36 Multiple, recent, and large epidemiological studies confirmed the beneficial effects of breast-feeding in reducing the risk of asthma, allergic rhinitis, and atopic eczema among children.^37–39

Probiotics, prebiotics, and synbiotics

Probiotics are defined as supplements that contain microorganisms (bacteria, yeast) and can change the microflora of the host.^40,41 The European Food and Feed Cultures Association and the International Life Sciences Institute defined probiotic as “a live microbial food ingredient that, when consumed in adequate amounts, confers health benefits on the consumers” and “live microorganisms that, when ingested or locally applied in sufficient numbers, provide the consumer with one or more proven health benefits”.^42,43 Probiotics might be effective in preventing antibiotic-related diarrhea in healthy children and necrotizing enterocolitis in very low-birth-weight infants, as well as preventing childhood atopy and treating acute viral gastroenteritis; moreover, probiotics might be beneficial in the treatment of irritable bowel syndrome, chronic ulcerative colitis, infantile colic, and Helicobacter pylori gastritis.⁴¹ The strains of probiotics most commonly used are Bifidobacterium and Lactobacillus spp.,^44,45 which are also well known to have the capacity of resisting the physicochemical environment of the digestive tract.⁴⁶

On the other hand, prebiotics are aliments or supplements that contain a variety of molecules that stimulate the activity and/or growth of the native probiotic bacteria; a common component of prebiotics is fiber/oligosaccharides.⁴¹ In the absence of fiber in the colon, anaerobic bacteria obtain their energy from protein fermentation, producing toxic metabolites, such as phenolic and ammoniac compounds.^47,48 By contrast, microbiota metabolites, such as short-chain fatty acids produce nontoxic material, such as propionate, butyrate, or acetate.^47,49 In a study conducted on rats, Gourgue-Jeannot et al⁵⁰ found that providing a fructan (inulin, fructooligosaccharide [FOS]) diet increased the synthesis of short-chain fatty acids. Currently, the most commonly used prebiotics are soybean oligosaccharides, FOS, GOS, and inulin.^51,52 Synbiotics are simply a mix of probiotic and prebiotic, which synergistically promote the growth of beneficial bacteria.^53,54

Use of probiotics in prevention of allergic diseases

Asthma

Several studies have been conducted using murine models to assess the effectiveness of probiotics in the treatment and prevention of asthma and hypersensitive airways. Blümer et al⁵⁵ studied the perinatal maternal administration of probiotics in mice. The study showed that application of Lactobacillus rhamnosus GG (LGG) suppressed significantly allergic airways and peribronchial inflammation in mouse offspring. In another murine model, Feleszko et al⁵⁶ found that application of either LGG or Bifidobacterium lactis (Bb12) significantly decreased pulmonary eosinophilia, airway reactivity, and antigen-specific IgE production. Moreover, the administration of a recombinant or wild-type Lactobacillus plantarum can effectively reduce airway eosinophilia following aerosolized allergen exposure.⁵⁷ Furthermore coadministration of L. plantarum and Lactococcus lactis has been shown to reduce allergen-induced basophil degranulation in a murine model of birch-pollen allergy.⁵⁸ Two different studies showed that oral administration of live Lactobacillus reuteri reduced hyperresponsiveness to methacholine, airway eosinophilia, and local cytokine responses.^59,60 There have also been several studies conducted on human beings, including children. Stockert et al⁶¹ found that using laser acupuncture and probiotics in school-age children with asthma might prevent acute respiratory exacerbations and decrease bronchial hyperreactivity. However, two studies showed no efficacy of oral L. rhamnosus administration to adolescents suffering from pollen allergy.^62,63 Moreover, a systematic review of RCTs showed that the effectiveness of probiotics for the treatment of allergic rhinitis and asthma is questionable.⁶⁴

Atopic dermatitis

Management with LGG may ameliorate eczema/dermatitis syndrome symptoms in IgE-sensitized infants, but not in non-IgE-sensitized infants.⁶⁵ In IgE-sensitized children, treatment with L. rhamnosus results in tangible reduction in the prevalence of any IgE-associated eczema.^65,66 Rosenfeldt et al⁶⁷ reported that the use of two Lactobacillus strains (rhamnosus and reuteri) in the treatment of allergic dermatitis was effective in children with increased IgE levels. In a review that included 13 RCTs, Betsi et al⁶⁸ concluded that regardless of IgE-sensitization, probiotics, especially LGG, can prevent atopic dermatitis. Vliagoftis et al⁶⁴ had the same conclusion in a meta-analysis of eleven studies. Furthermore, a meta-analysis showed that administration of probiotics during pregnancy reduced the risk of atopic eczema in children aged 2–7 years whose mothers received probiotics during pregnancy (reduction 5.7%, P=0.022).⁶⁹

Regardless of the timing of administration (pregnancy or early life), probiotics seem to prevent atopic dermatitis and IgE-associated atopic dermatitis in infants.⁷⁰ Han et al⁷¹ mentioned that probiotic L. plantarum CJLP133 supplementation is of good value in the treatment of pediatric atopic dermatitis. The preventive effect of LGG on atopic eczema has been also described in infants during the first 6 months of life.⁷² However, Kopp and Salfeld⁷³ presented a different opinion. In addition, Taylor et al⁷⁴ challenged the use of early probiotic supplementation with L. acidophilus, and found that it did not reduce the risk of atopic dermatitis in high-risk infants, but on the contrary was linked with increased allergen sensitization in infants receiving complements. In addition, Gore et al⁷⁵ found there was no usefulness from supplementation with B. lactis or Lactobacillus paracasei in the treatment of eczema, or benefits on the progression of allergic disease from age 1–3 years. Moreover, other authors found little evidence to support the idea of routine supplementation of probiotics to either infants or pregnant women to prevent allergic diseases in childhood.^76,77

Nonetheless, two major meta-analyses and one Cochrane study inferred that probiotics were not effective for the treatment for eczema.^78–80 Furthermore, RCT studies showed that the probiotic effect was not steady over the long term (4–7 years),^72,81 or even in the short term (6 months).⁸² Dotterud et al⁸³ conducted an RCT to assess the effect of perinatal administration of probiotics on childhood asthma or atopic sensitization. A total of 415 pregnant women were randomized to receive probiotics (LGG, L. acidophilus La-5 and B. animalis subsp. lactis Bb12) or placebo. At 2 years, there were no substantial effects on asthma or atopic sensitization in the group that received probiotics compared to placebo. The heterogeneity in results in different studies is perhaps attributable to different probiotic strains used, environmental factors, such as diet or geographic region, and genetic liability.^76,84

Allergic rhinitis

Probiotic treatment is beneficial in decreasing symptoms and reducing the use of relief medications in patients with seasonal and perennial allergic rhinitis.^64,73,85,86 Moreover, probiotic VSL3^® (Sigma-Tau Pharmaceuticals, Inc. Gaithersburg, MD, USA) can prevent the development of Parietaria major allergen-specific local and systemic response when administered intranasally to mice.⁸⁷

In a double-blind RCT, Giovannini et al⁸⁸ reported that long-term (12 months) consumption of fermented milk containing L. casei may improve the health status of preschool children with allergic rhinitis. Kuitunen et al⁸⁹ showed that probiotics can prevent IgE-associated allergy until the age of 5 years in cesarean-delivered children. Moreover, L. rhamnosus HN001 can protect against eczema and rhinoconjunctivitis when given in the first 2 years of life.⁹⁰ Furthermore, studies have shown that fermented milk prepared with Lactobacillus gasseri-, LGG-, and B. longum-supplemented yogurt can ameliorate nasal blockage and hence be effective in seasonal allergic rhinitis, such as in Japanese cedar pollinosis.^91–94

However, a meta-analysis reported no benefits of probiotic use for allergic rhinitis.⁷⁷ In addition, Tamura et al⁹⁵ concluded in their study that fermented milk containing L. casei strain Shirota was futile in preventing allergic symptoms, including Japanese cedar pollinosis.

Food allergy

It has been reported that oral intake of Lactobacillus or Bifidobacterium strains could improve food allergy.⁹⁶ In a murine-based study, oral administration of L. acidophilus AD031 and B. lactis AD011 prevented ovalbumin-induced food allergy in mice.⁹⁷ Other studies showed that administration of L. casei strain Shirota or VSL3 reduced anaphylaxis in a food-allergy model in mice.^87,97,98 Isolauri et al⁹⁹ mentioned in their study that supplementation of formulas with LGG ameliorated gastrointestinal symptoms in infants with eczema.

In contrast, administration of B. lactis Bb-12 and L. casei to extensively hydrolyzed formula did not improve cow’s milk tolerance in infants with cow’s milk allergy.¹⁰⁰ Moreover, other studies concluded that probiotics, specifically LGG or L. acidophilus, do not protect against cow’s milk allergy in infancy.^74,78,96,101 Finally, Osborn and Sinn⁷⁷ stated in their review involving 1,549 infants that the benefit of probiotics in improving food hypersensitivity is questionable.

Probiotic safety in disease prevention

Lactobacillus and Bifidobacterium as probiotics are considered to have a safe profile; however, there are limited data regarding the safety of other bacteria.¹⁰² The rate of systemic infection with probiotic strains has been reported as 0.05%–0.4% in a large study conducted on adults,¹⁰³ but it is very rare in infants and children.^104–106 Allen et al¹⁰⁷ reported that lactobacilli and bifidobacteria are well tolerated with no adverse effects during pregnancy and in infants. Probiotic products may include hidden allergens of food, and might not be suitable for patients with allergies to cow’s milk or hen’s eggs.¹⁰⁸ Other side effects of probiotics on children have been restricted to case reports. For instance, Robin et al¹⁰⁹ reported L. rhamnosus meningitis following recurrent episodes of bacteremia in a child undergoing allogeneic hematopoietic stem cell transplantation Moreover, Vahabnezhad et al¹¹⁰ reported Lactobacillus bacteremia associated with probiotic use in a pediatric patient with ulcerative colitis receiving systemic corticosteroids and infliximab, while Luong et al¹¹¹ reported a case of Lactobacillus empyema in an HIV-infected immunodeficient patient with a lung transplant receiving a probiotic containing LGG. Bacteremia due to Lactobacillus supplementation was also reported in an immunocompetent infant and child without gastrointestinal diseases.¹¹² Finally, antibiotic resistance could emerge due to the long-term use of probiotics under antibiotic-selection pressure, and the resistance gene could be conveyed to other bacteria.¹¹³

Use of prebiotics in prevention and treatment of allergic diseases

It has been postulated that prebiotics in infant formulas have the potential to prevent sensitization of infants to dietary allergens. In a 2-year follow-up RCT involving 132 infants at risk of atopy because of strong parental history, Arslanoglu et al¹¹⁴ reported that the cumulative incidences for atopic dermatitis, recurrent wheezing, and allergic urticaria were lower in the group of infants fed with a formula with either an added mixture of FOS and GOS compared to the placebo group. Furthermore, in a prospective, double-blind, and placebo-controlled fashion, a mixture of neutral prebiotic oligosaccharides was evaluated for protective effect against allergy. A total of 92 healthy infants at risk of atopy were fed either a placebo-supplemented (0.8 g/100 mL maltodextrin) hypoallergenic formula or prebiotic-supplemented (0.8 g/100 mL short-chain GOS/long-chain FOS) during the first 6 months of life. The authors concluded that feeding infants with oligosaccharide prebiotics (short-chain GOS/long-chain FOS) early in life protects against atopic dermatitis and allergic rhinoconjunctivitis.¹¹⁵

However, in a Cochrane study, Osborn and Sinn¹¹⁶ found that prebiotic supplementation in infant formula does not prevent food hypersensitivity or allergic disease. In addition, prebiotics alone are inferior to synbiotics in the treatment of moderate-to-severe childhood atopic dermatitis.¹¹⁷

Combined prebiotics and probiotics in the prevention of allergic disease

The effect of early intervention with synbiotics, a combination of probiotics and prebiotics, on the prevalence of asthma-like symptoms in infants with high risk of allergic diseases has been investigated. In a double-blind, placebo-controlled multicenter trial, 90 infants with atopic dermatitis, age <7 months, were randomized to receive an infant formula with GOS/FOS and B. breve M16V mixture or formula without synbiotics; 75 children completed the 1-year follow-up, and the study showed that the prevalence of wheezing was substantially lower in the synbiotic than in the placebo group.¹¹⁸ However, other studies showed that synbiotic has no benefit on the cumulative occurrence of allergic diseases, but was useful in reducing IgE-associated (atopic) diseases and the occurrence of atopic eczema.^41,119

Modified milk formulas during early life

Several health care practitioners recommend the use of soy-based formulas to treat infants with allergy or food intolerance. However, this is not currently recommended for prevention of allergy or food intolerance.¹²⁰ However, other formulas, such as hydrolyzed formulas, have been proposed for the prevention of allergy and food intolerance in infants.¹²¹ Hydrolyzed formulas consist of cow’s milk proteins that undergo enzymatic and chemical hydrolysis to decrease the peptide size, molecular weight, and allergenicity of the proteins.¹²² It has been postulated that when breast-feeding alone is not feasible in high-risk infants, partially hydrolyzed (casein- or whey-based) formulas are effective in preventing allergic diseases, particularly atopic dermatitis. In a systematic review, Alexander and Cabana¹²³ reviewed 18 articles to study the role of partially hydrolyzed 100% whey (pHF-W) protein infant formula in reducing the risk of atopic dermatitis. The study showed that feeding with pHF-W statistically significantly decreased the risk of atopic manifestations (summary relative-risk estimate 0.56, 95% confidence interval [CI]: 0.40–0.77). The authors concluded that exclusive breast-feeding should be the first choice of infant nutrition in the first 6 months of life. However, for infants who are not exclusively breast-fed, feeding with pHF-W instead of cow’s milk formula reduces the risk of atopic dermatitis in those with a family history of allergy. Moreover, Szajewska and Horvath¹²⁴ conducted a meta-analysis to study the evidence of a pHF-W formula in the prevention of allergic diseases. The authors concluded that for all allergic diseases and atopic eczema/atopic dermatitis, the use of the pHF compared with standard formula decreased the risk of allergy in high-risk children. However, in a Cochrane systematic review, Osborn and Sinn¹²¹ concluded that compared to exclusive breast-feeding, alimenting with hydrolyzed formula in the early days of infancy resulted in no significant difference in childhood cow’s milk allergy or infant allergy. With regard to extensively hydrolyzed (casein- or whey-based) formulas, the literature indicates that they are effective for primary prevention of allergy in high-risk infants, but they are not cost-effective when compared with partially hydrolyzed formulas.¹²⁵

Dietary patterns (Mediterranean diet)

Changes in dietary customs play an important role in the prevalence of symptoms of allergic diseases.^126–132 The Western diet constitutes a high intake of processed and red meat, fast food, sugary drinks, and full-fat dairy products, with minimal vegetables. This diet pattern contains high levels of polyunsaturated fatty acids (PUFAs) and ω6 fatty acids, which are risk factors for some chronic and allergic diseases.^133,134 Meanwhile, the Mediterranean diet comprises increased use of unrefined grain, fruits, and vegetables, moderate utilization of dairy products and milk, and low meat intake.¹³⁵ Several studies have mentioned the protective role the Mediterranean diet plays in some chronic diseases and allergies,^{133,135–138} perhaps due to its antioxidant and immunoregulatory properties.¹³⁹

For instance, de Batlle et al¹³⁷ conducted a cross-sectional study that included 1,476 Mexican children (6–7 years old). Nutritional data of the children’s intake in the previous 12 months and their mothers’ intake during pregnancy were gathered. The study showed that maintaining a Mediterranean diet was inversely associated with wheezing ever (odds ratio [OR] =0.64, 95% CI: 0.47–0.87), asthma ever (OR =0.60, 95% CI: 0.40–0.91), rhinitis ever (OR =0.41, 95% CI: 0.22–0.77), sneezing ever (OR =0.79, 95% CI: 0.59–1.07), and itchy/watery eyes (OR =0.63, 95% CI: 0.42–0.95). Chatzi and Kogevinas¹³³ echoed those results, and concluded that maintaining a Mediterranean diet in the early years can have a protective effect on the development of asthma and atopy in children. For older children, Arvaniti et al¹³⁶ included in his study 700 children 10–12 years old from 18 schools located in Athens, Greece. The authors concluded that adherence to a Mediterranean diet was inversely linked to the likelihood of asthma symptoms. Moreover, in a study conducted on adults showed that adherence to the Mediterranean diet reduced the risk of uncontrolled asthma by 78% (OR =0.22, 95% CI: 0.05–0.85; P=0.028).¹³⁹

In a meta-analysis and systematic review, Garcia-Marcos et al¹⁴⁰ studied the influence of the Mediterranean diet on asthma in children. The outcomes measured were prevalence of “current wheeze”, “current severe wheeze”, or “asthma ever”. The authors used ORs to compare the highest tertile of the scores with the lowest. In addition, random-effect meta-analyses for the whole group of studies were used and stratified by Mediterranean setting (centers <100 km from the Mediterranean coast). The study concluded that following a Mediterranean diet is associated with lower prevalence of “asthma ever” (OR =0.86, 95% CI: 0.78–0.95, P=0.004 [all]; OR =0.86, 95% CI: 0.74–1.01, P=0.06 [Mediterranean]; OR =0.86, 95% CI: 0.75–0.98, P=0.027 [non-Mediterranean]), “current wheeze” (OR =0.85, 95% CI: 0.75–0.98; P=0.02), driven by Mediterranean centers (OR =0.79, 95% CI: 0.66–0.94; P=0.009), and “current severe wheeze” (OR =0.82, 95% CI: 0.55–1.22, P=0.330 [all]; OR =0.66, 95% CI: 0.48–0.90, P=0.008 [Mediterranean]; and OR =0.99, 95% CI: 0.79–1.25, P=0.95 [non-Mediterranean]; with the difference between regions being significant). However, Tamay et al¹⁴¹ concluded in their large study that the protective effect of the Mediterranean diet on allergic rhinitis in elementary school children was not significant.

Micronutrients

As Hippocrates famously noted, “Let food be thy medicine”. Perhaps the factors with the greatest effect on modulating atopic expression are nutritional in nature. For those at risk, exposure to certain foods may contribute to severe, lifelong asthma or food allergies. Other foods, rich in anti-inflammatory antioxidants, may in fact ameliorate allergic responses to environmental stimuli. It is unlikely to be one factor that is solely responsible for unlocking genomic tendencies toward atopy.¹⁴²

An association between seasonal allergies and food allergies has been reported. In some individuals, allergic rhinitis due to specific pollens is connected to oral allergy symptoms due to certain fruits and vegetables.¹⁴³ Interestingly, organic versions of these same foods may be tolerated by patients who experience reactions with conventional produce. It may be that the pesticide stimulation of the immune system is responsible for this difference, or it may be that the level of antioxidant nutrients in organic foods is more concentrated.¹⁴⁴ There is no clear evidence on the role of vitamins A or C in primary prevention of allergic diseases. However, vitamin D in particular is of increasing interest in atopic prevention and treatment. A growing number of studies have consistently demonstrated that a majority of children are vitamin D-insufficient or -deficient. In children with asthma and allergic diseases, vitamin D deficiency is a strong correlate for asthma, allergic rhinitis, and wheezing.¹⁴⁵ Furthermore, research indicates that vitamin D deficiency functions as a strong predictor of asthma in children.^146–148

In a Cochrane study conducted by Bath-Hextall et al,¹⁴⁹ the authors assessed the following micronutrients once thought to be protective against eczema: sunflower oil (linoleic acid) versus fish oil versus placebo, docosahexaenoic acid versus control, oral zinc sulfate compared to placebo, vitamin D versus placebo, vitamin D versus vitamin E versus vitamins D plus E together versus placebo, pyridoxine versus placebo, sea buckthorn seed oil versus sea buckthorn pulp oil versus placebo, selenium versus selenium plus vitamin E versus placebo, and hempseed oil versus placebo. The study concluded that there was no conclusive evidence of the benefit of dietary supplements in eczema.

Essential fatty acids

More recent studies have looked at the role of essential fatty acids in reducing allergic disease. The evidence is very good for prenatal prevention of atopy when mothers ingest higher amounts of ω3 PUFAs.¹⁵⁰ It also appears that newborns who ingest breast milk relatively rich in ω3 are less likely to develop allergic symptoms.^151,152 This effect is most prominent in those babies at highest risk genetically for allergic diseases. Interestingly, the results of directly feeding infants PUFAs are not as clear. Studies of dietary modification with ω3 PUFAs in children at high risk demonstrated reduction in atopy.^153,154 Perhaps it is the balance of the two that is most important, and one must also take into account preexisting dietary deficiencies and genomic factors. More research is clearly needed in this realm before universal recommendations can be made.

Conclusion and recommendations

Multiple, recent, and large epidemiological studies have confirmed the beneficial effects of breast-feeding in reducing the risk of asthma, allergic rhinitis, and atopic eczema among children. Breast milk constitutes a major source of support to gut colonization, due to its bifidobacteria content and by providing a large amount of GOS, which selectively accelerate the growth of bifidobacteria. The literature lacks consensus in recommending the addition of probiotics to foods for prevention and treatment of allergic diseases. Moreover, probiotics must not be provided to immunocompromised children. Despite many studies portraying safety and efficacy in the use of probiotics during pregnancy and lactation, more evidence is required before a strong recommendation can be made. Probiotics in infant formula are safe. However, more RCTs are needed to compare human milk versus infant formula supplemented with probiotics.

Prebiotics may prove to be effective in reducing atopy in healthy children. Prebiotics in infant formula have been found to be safe, but clinical efficacy has to be investigated more, not to mention the high cost burden on families. There is insufficient evidence to support soy formulas or amino acid formulas for prevention of allergic disease. Partially hydrolyzed formulas are of use in infants at high risk of atopic dermatitis in the first 6 months of life instead of intact cow’s milk protein formula. A healthy diet, such as the Mediterranean diet, may have a protective effect on the development of asthma and atopy in children. In children with asthma and allergic diseases, vitamin D deficiency is a strong correlate for asthma, allergic rhinitis, and wheezing. It is worth mentioning that the current deficiency of evidence of efficacy does not mean that future clinical investigations will not establish tangible health benefits for probiotics, prebiotics, diet, and vitamin supplementation.

Acknowledgments

The authors would like to thank Hamad Medical Corporation for the support and ethical approval (HMC Research Protocol No. 15455/15).

Disclosure

The authors report no conflicts of interest in this work.

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